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A toxic legacy

Two studies came out in the last couple of weeks that really illustrate the problems associated with "PBT" chemicals: those which are simultaneously persistent, bioaccumulative, and toxic. Persistent substances resist degradation — you can move them around but it's really hard to get rid of them. Bioaccumulation happens when chemicals in food, water, and air end up getting stored in the body of a living thing. Thus, for a bass living in a mercury polluted lake, the mercury levels in the fish may be thousands of times higher than the levels in the water. A cow grazing on PCB-laced feed will store the chemical in her body and excrete it in her milk, and humans too act as sinks for all kinds of chemicals.

And toxic means, well, toxic, but it's good to keep in mind that many PBTs are carcinogens and developmental toxins, meaning that years may elapse between exposure and the manifestation of health harms.

"Environmental Chemicals in Pregnant Women in the US," which came out January 14 in Environmental Health Perspectives (EHP), documents the PBT problem in expectant mothers. The researchers analyzed blood and urine samples from 200+ pregnant women for 163 different chemicals, and found most of them in most women. Not all of these are PBTs. For example cotinine, a nicotine metabolite, is cleared from the body quickly. But since smoking in public is still common, most pregnant women are continually exposed to nicotine. But for many other chemicals their occurence is all about bioaccumulation and persistence. DDT, for example, was banned in the U.S. almost 40 years ago — yet it was found in 2/3 of the subjects; its metabolite DDE was found in each and every women. In fact, of all the chemicals measured in blood fat, the levels of DDE were the highest.

"Assessment of Non-Occupational Exposure to DDT in the Tropics and the North," also recently published in EHP, showcases the 'P' and 'B' of DDT as well as its potential for "LTR" or Long Range Transport. The researchers looked at DDT intake and blood levels for four groups: people living in homes where DDT has been sprayed to control malaria vectors, people living in non-sprayed homes in Africa and the tropics, people in northern regions (e.g., Alaska, Canada, Scandanavia), and finally Inuits living in Greenland. The good news is that blood levels show a steady decline since the 1960s, when DDT was used liberally in agriculture, forestry, and home pest control. And, as you might expect, the group with highest levels are those people living in homes where DDT is still used. But the study also shows that DDT intake remains high, even for people living in areas where DDT hasn't been used for decades — or ever. And in the tropics, daily intake can exceed the World Health Organization's guidelines for levels of concern.

Perhaps the most interesting finding is that Inuit Greenlanders experience the same degree of DDT exposure as people living in the tropics — 12 times higher than non-Inuit northerners. This extra burden is due to their indigenous diet that includes marine mammals. Just as DDT bioaccumulates in individual animals, it also biomagnifies across food chains, and Inuits sit at the top of a very long food chain.

Neither of these studies explicitly address the 'T' of PBT — toxicity — but the human health harms of DDT and most of the other chemicals found in pregnant women are well documented.

For me, these studies clearly underscore the need to end our dependence on PBTs. The use of DDT peaked in the U.S. in the late 50's and we banned it in 1972, yet it still contaminates the bodies of most of us. But DDT is not unique in its ability to stick around, and that makes me think about the PBTs we are still using, like the polybrominated flame retardants, which were found in almost of the all women in the first study. Or lindane, which was not tested in this study, but which is still used to treat lice in the U.S. Or endosulfan (also not tested for), which is being phased out, but on an extended, six-year time line. It will take many years, perhaps decades, for chemicals like these to leave our bodies and the bodies of our children. So it's imperative that we stop using and producing them — and sooner rather than later.